Evaluating the Role of Triborane(7) As Catalyst in the Pyrolysis of Tetraborane(10)

2013-09-19T00:00:00Z (GMT) by Baili Sun Michael L. McKee
The initial steps in the B4H10 pyrolysis mechanism have been elucidated. The mechanism can be divided into three stages: initial formation of B4H8, production of volatile boranes with B3H7 acting as a catalyst, and formation of nonvolatile products. The first step is B4H10 decomposition to either B4H8/H2 or B3H7/BH3 where the free energy barrier for the first pathway is 5.6 kcal/mol higher (G4, 333 K) than the second pathway when transition state theory (TST) is used. When variation transition state theory (VTST) is used for formation of B3H7/BH3, the two pathways become very similar in free energy with the B4H8/H2 pathway becoming favored at G4 by 1.0 kcal/mol at 333 K (33.1 versus 34.1 kcal/mol). The experimental activation energy for B4H10 pyrolysis is about 10 kcal/mol lower than the calculated barrier for B4H10 → B4H8 + H2, which indicates that this reaction is not the rate-determining step. We suggest that the rate-determining step is B4H10 + B3H7 → B4H8 + H2 + B3H7 where B3H7 acts as a catalyst. The role of reactive boron hydrides such as B3H7 and B4H8 as catalysts in the build-up of larger boron hydrides may be more common than that previously considered.